The Stovebolt Site Forums The Parking Lot '60-'66 Trucks '65 C20 four speed ID

I recently purchased a C20 with a manual 4 speed. I would like to change the gears in the tranny to get rid of the granny first gear and possibly turn fourth into an overdrive. I've not been able to positively identify my transmission but it looks like the 4 speed depicted in the '63 Service Manual. Any help would be greatly appreciated.

Welcome aboard!

The (RPO) MY6 from an 82-86, sounds like what you are looking for. The MY6 is a New Process A833 designed for Chrysler and used behind everything from the slant 6 to the Hemi, and in the 3/4 Dodges. Fourth gear is an overdrive. The MY6 will bolt right in. The bad news is you will end up with a floor shift just in front of the seat, and the drive shaft will have to be shorten about 9". Another option would be the T-5, fifth gear overdrive. The t-5 wouldn't be good for hauling a serious load.

I have the MY6 in my 63 C20, and like it. The gears are comparable to 2-4 of the M420 which you have.

Larry

You most likely have the SM 420 as mentioned. Your idea sounds good, but it probably won't work. They were converting Muncie 4-Speeds for a while and realized there were problems with it. When not needing the Overdrive and cruising around in 4th gear (the old 3-Speed position) it was wearing out bearings too quick. The trans was only in the "Direct Drive" position when in Overdrive. Besides that, I don't believe that anyone is making the gears that you would need. Someone with trans rebuilding experience could probably verify this, but I believe you would be better off with something like what Larry suggested.

Bill

Welcome to the site, by the way.

Bill

Thanks for the info guys. I had suspected that I had a SM 420 but the guy I bought the truck from thought it was a Muncie (are they the same thing?). He also said that he had found an "Italian Gear Set" online that would convert the tranny into a more useful 4 speed by getting rid of the granny gear (I don't plan on pulling any stumps in the near future). But he couldn't remember on which website he had found the gear set. I am planning on making this my daily driver to and from work, about 20 miles each way, mostly at highway speeds. Again, thanks for the info and the welcome.

Richard

For a quick fix, you could put taller tires on the rear. It won't be a world of difference, but it should be notice-able. I put extra tall tires on my '72 C-10 to help with the 3.73's and it seemed to knock it down to about 3.63.

Bill

Thanks Bill, I'll keep that in mind.

Richard

The italian geasets are available........for Muncies. And no, your SM 420 isn't a Muncie. It's a Saginaw. Lots of terms have been thrown around for years by those who don't know any better, and those that should. Believe me, if a simple overdrive fix for an SM420 is possible, it would have been done by now.
You could buy an expensive overdrive like a gear vendors. or a less expensive overdrive like a Ranger overdrive. Or maybe you could luck out and find an old Spicer high/low box (2021)
But the path of least resistance is most likely the T-5, NV4500, A833 avenue. Plenty of info on the interweb and good availability of units.

Thank you for the information. I was just looking on Ebay at T5 transmissions and it appears to me that using one of these would require a lot of serious modification to the cab and/or drivetrain. The shifter looks like it would probably come up through the bench seat. Has anyone tried this modification?

Richard

"Has anyone tried this modification?" HAHAHA! Probably just a few hundred guys on here and a few hundred more on sites like OldGMCtrucks.com and the HAMB and 67-72 Chevy trucks and and and......... So dial up your search fingers, pull your pants up and get yourself some knowledge. And the shifter problem is real but can be solved. Start reading.

Might as well ask my question bout my SM-318 3speed while it's related..wanting to keep stock equipment but not sure what kinda gears mine has since it isn't quite running yet, almost positive it's a 3.07 open diff. rear end. anyone got ideas how to go about highway cruising gears in the stock box?

I would think a 3.07 would be considered a highway gear. I drove a F@^& F250 daily for 5 years with a 4 speed NP435 and a 4.10 rear end. Yes it was churning 4000 rpm at 80 mph, but I drove it from OKC to dallas and back that way many times. Of course it only got 7 mpg but I never had any major problems. On the other hand, my 2006 Silverado with the 5.3 liter engine has a 3.23 rear end and will barely get out of it's own way from a stop. It runs under 2000 rpms at 70 in overdrive. If I had an older truck with a 1:1 direct drive top gear, I'd want a 3.43-3.73 ratio for good acceleration and highway speed. As far as the 4 speed goes, I'd keep it and change the rear axle (I'm a truck driver so I appreciate that extra low gear!)